Anti-reflection films are attached to various display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescence display (ELD) or a cathode ray tube (CRT).
Conventional anti-reflection films have two or more transparent layers made of metal oxides superposed one on another. The transparent layers are used to effectively lower reflection of light of wide wavelength region. The metal oxide transparent layer has been formed according to a chemical vacuum deposition (CVD) method or a physical vacuum deposition (PVD) method, particularly the PVD method. The metal oxide transparent layer has an excellent optical characteristic as an anti-reflection film. However, the deposition method is not suitable for mass production because the productivity of the method is low.
A coating method has recently been proposed in place of the deposition method. Optical functional layers are coated on a transparent support according to the coating method.
A low refractive index layer (having a refractive index lower than a refractive index of a transparent support) should be provided to obtain an anti-reflection film. In the case that two or more optical functional layers are provided on a transparent support, the low refractive index layer is arranged as the uppermost layer. The low refractive index can effectively be obtained by introducing voids into the low refractive index layer. A layer containing voids (i.e., air) has a low refractive index because the refractive index of the air is 1.00. Micro voids can be formed in the low refractive index layer by adding particles into the layer. The micro voids are present between the particles or contained in the particles.
Japanese Patent Publication No. 60(1985)-59250 discloses an anti-reflection layer having micro pores and inorganic particles. The anti-reflection layer is formed according to a coating method. After coating the layer, the layer is subjected to an activated gas treatment. The gas is emitted from the layer to form the micro pores.
Japanese Patent Provisional Publication No. 2(1990)-245702 discloses an anti-reflection film comprising two or more fine particles (such as MgF.sub.2 and SiO.sub.2) in which blending ratio of the particles is changed along a thickness direction. A refractive index is adjusted by the blending ratio to obtain an optical effect of an anti-reflection film having a high refractive index layer and a low refractive index layer. The fine particles are adhered with SiO.sub.2, which has been formed by thermal decomposition of ethyl silicate. Carbon dioxide and vapor are also formed by burning an ethyl moiety of the ethyl silicate at the thermal decomposition. Carbon dioxide and vapor are emitted from the layer to form micro voids between the particles, as is shown in FIG. 1 of the publication.
Japanese Patent Provisional Publication No. 7(1995)-48527 discloses an anti-reflection film comprising inorganic powder of porous silica and a binder.
Japanese Patent Provisional Publications No. 9(1997)-288201 discloses an anti-reflection film having a low refractive index layer in which micro particles of a fluoropolymer are superposed upon each other to form micro voids surrounded by the particles.